Turning tests of 2011 (T8) aluminum alloy were carried out and an attempt to evaluate the free-machining properties of the alloy was made under the following conditions: cutting speed: 25300m/min., feed: 0.020.32mm/rev., and depth of cut: 13mm with high speed steel tools having the side rake angle of 0, 10, and 30. It has been observed that the number of chips (N) contained in 100g of cut varied from 1 to 106, according to the cutting conditions. At a constant speed, N decreased to the mininum of 1 (corresponding to the formation of stringy chips) with increasing feed, and then increased again to reach a nearly constant value. Thus, when N was plotted against feed, there appeared a discontinuity, i. e., there existed some critical values of feed for the formation of discrete chips. The range of feed for forming continuous flow type chips became wider with the increase of the cutting speed and more markedly with the increase of the side rake angle. Chips were always broken at a rvey low cutting speed andat a small side rake angle.Based on the above observations, procedures for explaining the machinability of 2011 aluminum alloy was discussed in terms of the critical feed. It was proved that the number of chips for a constant amount of the material cut can be adopted as a criterion of machinability from the standpoint of chip treatment.